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In this paper a very compact device of a very efficient high- power fiber-laser based on a specially designed Ytterbium (Yb)-doped double-clad D-Shape silica fiber with an output power of more than 8 watts will be described. The development, preparation and characterization of rare-earth doped double- clad optical fibers based on silica for the application in high-power fiber-lasers were done at the Optics Division of the Institute for Physical High-Technology e.V. Jena (IPHT) in the last few years. The doping with Ytterbium as the laseractive component was one of the most important questions which were investigated. Many samples had to be made for the optimum concentration of Yb, the optimum codoping and geometry, respectively. As a result of these experiments we realized a double-clad fiber with D-shape geometry of the pump-core. This D-shape resulted from ray-tracing calculations performed at the Lasercentre Hannover e.V. (LZH) as an optimal solution for the high conversion efficiency of the pump-power. Considering these numerical results, such fibers were realized at IPHT by drawing a sidepolished preform by preserving the cross section of the preform. After some basic investigations on the optimum wavelength for the pump-light and the resonator length, respectively, a compact device with a launched pump- power of approx. 13 W and an output-power of more than 8 watts was developed and realized. The maximum output-power was only limited by the pump-power available at our laboratory at the chosen wavelength. Main advantages of the setup are the short length of the laseractive fiber (less than 10 m), the high efficiency (greater than 60%) and the compactness of the device. Additionally, the diameter of the pump-core was reduced to 125 micrometer. These kinds of fibers were tested with a specially designed pump-source of the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena. The first result with a double-clad Yb-doped fiber of only 4 meter length was a fiber-laser with an output-power of about 4 Watts. Last but not least, some basic experiments with Fiber-Bragg-Gratings (FBG) as one of the resonator mirrors were done. A fiber-laser with a FBG for the laser-wavelength had an output-power of about 700 mW with a FWHM of about 0.1 nm.
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